The invention relates especially to power supplies which are implemented on the circuit board of an electronic device. The circuit board typically contains a great number of other circuits, with the power supply located on the board providing the supply voltage for said circuits. The power of such power supplies is typically 2 to 100 W. One operation environment is formed by telecommunication devices with subracks often containing several parallel circuit boards, each of which requires a separate power supply.
In power supply circuits located on a circuit board, which are typically switched mode power supplies, the planar transformers are traditionally implemented as discrete components in which thin winding plates manufactured, for example, from copper plate or folio are stacked alternatively with insulator folios in a stack around which is attached a ferrite core. The drawback of this kind of traditional structure is its price; the manufacture of the layers and their stacking into a stack, and equipping the component with connectors for connection to the circuit board make the price of a single transformer high.
This drawback has been eliminated in new transformers in which the transformer windings are integrated on the same circuit board as that on which the power supply has been implemented. This kind of structure is illustrated in FIG. 1, which shows four conductive layers A to D of a multiple layer circuit board PCB1, on each of which has been implemented the desired winding pattern WP1 to WP4. These winding patterns form together the windings of a transformer (primary and secondary windings). In the example shown in the figure, the transformer core is formed of the ferrite pieces F1 and F2 with an E profile. For the ferrites the circuit board includes holes H1 to H3 which go through the board at the location of the arms of the E. The ferrite pieces are pushed from the opposite sides of the circuit board into the holes, whereby they settle in the holes against one another so that the end surfaces of the arms of the E settle against one another. After this the pieces are locked to the circuit board by using a suitable mechanical locking piece.
Such a structure which has been integrated to the circuit board has, however, current tolerance and efficiency problems caused by the fact that the structure of the transformer, especially the thickness and number of the winding layers, is dependent on the circuit board on which the power supply has been implemented. The layer thickness of a multiple layer circuit board is nowadays already unduly thin (typically 35 .mu.m) when considering the power required from the power supply, and it can be predicted that the layer thicknesses will decrease even further. In this kind of structure the conductors inevitably become very thin, which causes low efficiency in the power supply, especially when using high power.
These problems can be solved by using a transformer structure in which part of the transformer windings are implemented on a separate substrate (piece of circuit board) which is placed on the circuit board (mother board) on which the rest of the transformer windings and the rest of the components (among others, the power supply) are implemented. This kind of solution has been presented, for example, in U.S. Pat. No. 5,321,380 and EP patent application 318 955. By using this kind of basic structure it is possible to place the secondary winding on a separate substrate, for example, because the current is usually higher in the secondary winding than in the primary winding.
The drawback of such a structure is, however, that the assembly of the transformer on the mother board comprises several work phases which are not compatible with the automated manufacturing process used to place and solder the other components on the mother board. The introduction of the work phases required by the transformer assembly to the manufacturing process of the mother board thereby makes the manufacture of the mother board significantly more difficult.